In general, various kinds of electrolytes have been used in electrochemical devices widely used in recent years, such as lithium secondary batteries, electrolytic condensers, electric double layer capacitors or electrochromic display devices, and dye-sensitized solar cells that have been studied intensively for their practical use in the future. Under these circumstances, importance of such electrolytes has increased more and more.
Electrolytes that have been used the most widely in recent years include non-aqueous solutions containing electrolyte salts dissolved in organic solvents such as ethylene carbonate, propylene carbonate, dimethoxyethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofuran or acetonitrile. Such non-aqueous electrolytes have to serve as media performing conduction of ions and electrons between both electrodes, and should be stable in the drive voltage range of a device to which they are applied and show sufficiently high ion conductivity. However, the organic solvents used in such electrolytes have a low viscosity, leak out easily, and have high volatility to cause evaporation. Additionally, such organic solvents are ignitable, and thus are problematic in terms of long-term reliability, durability and stability. Therefore, recently, many studies have been conducted to utilize an ionic liquid as an electrolyte for an electrochemical device. However, conventional ionic liquids are expensive, and are obtained via a complicated preparation and purification process. Moreover, a liquid electrolyte is disadvantageous because it has possibility of leakage and it is not applicable to devices designed to have a large size or a small thickness.
Meanwhile, G. Berchiesi and coworkers have prepared a eutectic mixture by using acetamide, LiSCN (lithium thiocyanate), CH3COOLi (lithium acetate), or the like (Thermochimica Acta, 1983, 70, pp 317-324). However, there is no disclosure of the use of the above eutectic mixture as an electrolyte for an electrochemical device.